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Influence of adrenoceptor and muscarinic receptor blockade on the cardiovascular effects of exogenous noradrenaline and of endogenous noradrenaline released by infused tyramine

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This study aimed firstly to compare the in vivo cardiovascular effects of exogenously administered and of endogenously released noradrenaline; secondly to characterize the adrenoceptors mediating these responses; thirdly to assess the influence of parasympathetic tone on the cardiovascular effects of noradrenaline. In two randomised placebo-controlled studies, healthy, young, male volunteers received intravenous (i.v.) infusions of noradrenaline at six incremental doses of 10–160 ng/kg/min and – in order to release endogenous noradrenaline – tyramine at four incremental doses of 5–20 μg/kg/min. Noradrenaline and tyramine were administered in the absence and presence of α1-adrenoceptor blockade with doxazosin (2 mg p.o.), α2-adrenoceptor blockade with yohimbine (15 mg p.o.), selective β1-adrenoceptor blockade with bisoprolol (15 mg p.o.) and muscarinic receptor blockade with atropine (15 μg/kg i.v. loading dose followed by 0.15 μg/kg/min by i.v. infusion). Vasoconstrictor effects were assessed by measurement of diastolic blood pressure (Pdiast) and myocardial effects by measurement of systolic time intervals, namely the duration of electromechanical systole corrected for heart rate (QS2c). I.v. noradrenaline increased Pdiast (Δmax 17 mmHg) and this was nearly completely suppressed by doxazosin but only slightly blunted by yohimbine. Noradrenaline also slightly shortened QS2c (Δmax –22 ms), and this was potentiated by both doxazosin and yohimbine and completely blocked by bisoprolol. I.v. tyramine reduced Pdiast (Δmax –7 mmHg), which was not affected by α1-adrenoceptor blockade, and profoundly shortened QS2c (Δmax -104 ms) which was significantly correlated with a marked increase in systolic blood pressure (Psyst) (Δmax 57 mmHg). The shortening of QS2c and the rise in Psyst were not influenced by α-adrenoceptor blockade but were antagonized by bisoprolol. Atropine potentiated the blood pressure rise and the shortening of QS2c induced by i.v. noradrenaline and converted the fall in Pdiast induced by i.v. tyramine into an increase. Thus the cardiovascular effects of exogenous noradrenaline are mainly characterized by α1-adrenoceptor-mediated vasoconstriction and the actions of endogenous noradrenaline (released by i.v. tyramine) by β1-adrenoceptor-mediated positive inotropic effects. The rise in Psyst with i.v. tyramine most likely reflects positive inotropism and not a vascular ‘pressor’ response.

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Received: 27 June 1996 / Accepted: 25 October 1996

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Schäfers, R., Poller, U., Pönicke, K. et al. Influence of adrenoceptor and muscarinic receptor blockade on the cardiovascular effects of exogenous noradrenaline and of endogenous noradrenaline released by infused tyramine. Naunyn-Schmiedeberg's Arch Pharmacol 355, 239–249 (1997).

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